1. Field of the Invention
The present invention relates to the determination of properties of an optical device under test, e.g. the determination of the group delay of the optical device. The group delay is a fundamental property of optical devices, such as single mode optical fibers or optical components such as Bragg gratings which devices are used in the optical transmission of information.
2. Discussion of the Background Art
As for the background of optical transmission of data it has to be said that the premier feature of optical fiber is its extremely low loss. This has made it the dominant transmission medium for long link lengths. The loss characteristics of fiber determines were optical communication is practical. At 1550 nanometer single mode optical fiber has an attenuation of 0.2 dB/km. This allows fiber optic signals to be propagated through very long length of fiber without regeneration. Telecommunication systems use the 1300 and 1550 nm windows for lowest loss in the fiber. Since a telecommunication system must cover a very large distance, the aforementioned attenuation of the single strength in the fiber is of high importance. Therefore, the loss characteristics of optical fiber often limit the distance that a signal can propagate in the fiber.
However, this is not always the case. In single mode fibers, chromatic dispersion can limit the distance over which fiber optic signals can propagate. Chromatic dispersion describes the fact that the speed of signal propagation in the fiber depends on the wavelength of the light. The consequence is that as the signal propagates through a long length of fiber, the edges of the waveform of the signal start to become more rounded. Eventually, the adjacent bits start to overlap in time causing the digital waveform to have poor readability. The amount of signal rounding depends beyond other parameters on the amount of chromatic dispersion in the cable. The problem of this pulse spreading is a problem in today""s fiber communication because the increasing bit rate of the state of the art fiber communication systems, which bit rate reaches numbers of up to 40 Gbit/s per channel, brings the chromatic dispersion in the range of the bit resolution of such a 40 Gbit/s communication system.
To compensate chromatic dispersion in a fiber chirped Bragg gratings were developed. In these gratings different wavelengths of the pulse need different amounts of time traveling through the grating to compensate the dispersion caused by the fiber. The concept of this compensation is schematically illustrated in FIG. 1. FIG. 1 shows the original pulse 1 that is broadened to a spreaded pulse 2 while traveling through a fiber 3. With the help of a circulator 4 a chirped Bragg grating 5 is introduced in the path of the pulse 2. As described before the resulting pulse 6 has the original shape of pulse 1 again.
However, to use such a component as a chirped Bragg grating it is necessary to have exact knowledge of the chromatic dispersion caused by such a grating. The measurement of chromatic dispersion is accomplished by analyzing the group delay through the fiber as a function of wavelength. To measure the group delay a wavelength tunable optical source such as a tunable laser is used to generate coherent light at different wavelengths. In the prior art the wavelength of the tunable laser is then incremented step by step and for each wavelength step a group delay is determined. Finally, the group delay details are used to calculate the chromatic dispersion coefficient. The disadvantage of measuring the group delays step by step is the time, which is needed to perform all the wavelength steps and the respective measurements. In other words this means it takes quite a long time to get data, which are so precise that this precision is sufficient for the needs of the telecommunication industry.
Therefore, it is an object of the invention to provide an improved determination of a property of an optical device under test, e.g. the group delay of the optical device. The object is solved by the independent claims.
The main advantage of the present invention is that it is possible to measure the group delay and therefore the chromatic dispersion of an optical device, such as a fiber or a chirped Bragg grating, in a short amount of time while still keeping the precision of the data high enough for the needs of the telecommunication industry. This goal is reached by the invention by tuning the frequency of the coherent light beam of the laser from a maximum to a minimum of a given frequency range in a given time interval, e.g. making a wavelength sweep through the given wavelength range. Because of the inventive correction of effects caused by a non-linearity in the tuning of the laser this sweep can be done without detrimental broadening of the resulting spectral width. Method and apparatus of the invention avoid the aforementioned problems of the prior art and provide for exact data well keeping the measurement time low.
In a preferred embodiment of the invention there is performed a filtering of the Fourier transformed first signal with a high pass filter. It is further preferred to use a Hanning window as a shape for the high pass filter. Using this filter makes sure that a good elimination of not usable data is possible while still having important parts of data at the edges of the spectrum within the filter. Moreover, it is preferred to use a half of a Hanning window as a shape for the high-pass filter.
In a further preferred embodiment of the invention this high pass filter is adapted to the precision and the shape of the resulting spectrum of the signal corrected for a non-linearity of the laser by making an interferometric signal out of the corrected first phase signal, Fourier transforming the interferometric signal to get a spectral signal, determining a fraction of the maximum of the spectral signal, determining the abscissas of the intersections of the ordinate of the fraction which the curve of the spectral signal, determining the mean frequency fmean as the average of the abscissas, band pass filtering the spectral signal with a band pass filter having its center at the mean frequency and having a width greater than the width of the frequency range. The inventive concept of adaptive filtering of the signal corrected for a non-linearity of the laser is doubling the inventive success since by eliminating the non-linearity the resulting peak of the Fourier transformed spectrum is very sharp so that it can be used a filter width which is very much smaller than known filter width in the prior art. As can be seen in the following detailed description of the invention and the respective drawings this inventive concept is resulting in very smooth curves showing the group delay.
In a preferred embodiment the inventive method comprises the following steps:
splitting the first initial light beam into a first light beam and a second light beam,
coupling the first light beam into the optical device under test,
letting the second light beam travel a different path as the first light beam,
superimposing the first and the second light beam to produce interference between the first light beam and the second light beam in a resulting first superimposed light beam,
detecting as a first signal the power of the first superimposed light beam as a function of time when tuning the frequency of the coherent light beam from a minimum to a maximum of a given frequency range in a given time interval,
splitting the second initial light beam in a third light beam and a fourth light beam,
superimposing the third light beam and the fourth light beam after each light beam has traveled a different path, to produce interference between the third and the fourth light beam in a resulting second superimposed light beam,
detecting as a second signal the power of the resulting second superimposed light beam as a function of time when tuning the frequency of the coherent light beam from a maximum to a minimum of a given frequency range in a given time interval,
using the detected second signal for deriving a non-linearity information about a non-linearity in a tuning gradient of the frequency when tuning the frequency of the coherent light beam from the maximum to the minimum of the given frequency range, and
using the non-linearity information for correcting effects on the first signal caused by the non-linearity to get a corrected first signal.
It is further preferred to flatten the first and/or the second signal before transforming it by approximating it with polynoms of at least second order.
Additionally it is preferred to tune the frequency with a tuning velocity of about 5 nm/s.
Additionally it is preferred to tune the frequency within a tuning interval of about c/1400 nm to c/1600 nm, c=velocity of light.
Additionally it is preferred to detect the power at a rate of about 100 kHz.
The term xe2x80x9ccoherentxe2x80x9d in this application means that the coherence length of the light beam is bigger than the difference of the path of the first and second and third and fourth light beams, respectively.
Other preferred embodiments are shown by the dependent claims.
It is clear that the invention can be partly or entirely embodied or supported by one or more suitable software programs, which can be stored on or otherwise provided by any kind of data carrier, and which might be executed in or by any suitable data processing unit.